1. Field of the Invention
This invention relates to a motor for successively applying currents to respective phase windings by the ON/OFF operations of switching elements. More particularly, the invention is related to an apparatus for reducing motor driving noise, which is designed to reduce driving noise caused by the switching of a current applied to each phase winding, and also a method for generating a motor driving noise reducing signal.
2. Description of Related Art
FIG. 22 shows an example of the driving circuit of a 3 phase brushless motor 101. In the example, each of the paired transistors Q1 and Q2, Q3 and Q4, and Q5 and Q6 as switching elements are connected in series between a power supply and a ground, wherein one end of each of the transistors Q1, Q3 and Q5 is connected to a power supply terminal, and one end of each of the transistors Q2, Q4 and Q6 is grounded through a current sense resistor 102, and wherein one end of each of the star-connected phase windings 101U, 101V and 101W of the motor 101 is connected to each of the connection points P1 P2, and P3 of each of the paired transistors Q1 and Q2, Q3 and Q4, and Q5 and Q6.
FIG. 23 shows a circuit for generating a driving signal to be supplied/applied to each of the above-described transistors. In the drawing, a reference numeral 104 denotes an input terminal for receiving the entry of 120xc2x0 current application control signals LEU, LEV and LEW (referred to as LE or LE signal, hereinafter) and 180xc2x0 current application direction deciding signals INU, INV and INW (referred to as IN or IN signal, hereinafter) outputted from a pulse signal generating circuit, not shown; 105 a PWM waveform signal generator for generating a PWM waveform signal; 106 a PWM synthesizer for synthesizing the PWM waveform signal and the IN signals; and 107 an output terminal for outputting the LE signals and the IN signals outputted from the PWM synthesizer 106, specifically outputting the LE signals and the IN signals of PWM waveform patterns shown in FIG. 24. These six kinds of signals (current application control signals LEU, LEV and LEW, and current application direction deciding signals INU, INV and INW) have meanings as shown in FIG. 32.
Each of FIGS. 25A to 25F show a state that the motor is driven by applying the IN signals of the PWM waveform pattern to the ground side transistors Q2, Q4 and Q6 (low side), and applying the LE signals of the PWM waveform pattern to the power supply side transistors Q1, Q3 and Q5 (high side), and thereby successively supplying currents to the respective phase windings 101U, 101V and 101W of the motor 101 by every 60xc2x0. In FIGS. 24 and 26, com 1 to com 6 show the states in which currents are supplied to the windings 101U, 101V and 101W by timings in the case of 120xc2x0 current application, and the timings are shown in FIGS. 25A to 25F. In this case, since a square-wave current flows to each of the phase windings 101U, 101V and 101W, sudden fluctuation occurs in torque, and the motor is vibrated to generate noise. This noise causes vibration by an integral multiple of the natural vibration of a casing covering the motor, and is generated in an audible band.
As explained above, the PWM driving is designed to switch a voltage applied to the motor 101, substantially between two ways, i.e., a power supply voltage and 0V, and control the amount of current flowing to the motor to be average based on the duty thereof Consequently, the current waveform of the PWM driving becomes a square current waveform, causing the generation of noise.
Then, as shown in FIG. 27, a signal for increasing the current of a duty waveform pattern and a signal for reducing a current are outputted from a noise reducing waveform signal generator 108. The signal for current increase and the signal for current reduction are selected by an INC/DEC selector 109 based on a COMOUT signal as a commutation control signal, and synthesized into the LE signals of PWM waveform patterns shown in FIG. 28 by a signal synthesizer 110. Then, the signals shown in FIG. 29 are applied to the transistors Q1, Q3 and Q5 of the motor driving circuit 101, and the currents like those shown in FIG. 30 are supplied to the respective phase windings. Accordingly, the current change becomes continuous during commutation control, causing no sudden torque changes. Therefore, the noise generated by the motor vibration is reduced.
However, when currents are supplied to the motor windings 101U, 101V and 101W by simply applying the IN and LE signals shown in FIG. 29 respectively to the transistors Q1, Q4 and A6, and the transistors Q1, Q3 and Q5, phases at points A to F shown in the same drawing may be short-circuited from one another depending on PWM timings.
FIG. 31 illustrates a state where phases are short-circuited from one another. A short brake is set in when the transistors Q1, Q3 and Q5 of FIG. 31 are simultaneously turned ON. The timing thereof is such that when the U phase turns On the power supply side transistors Q1, and the W phase is regenerated (transistor Q6 is OFF, while transistor Q5 is ON) to control a current, the current is supplied to the V phase earlier than a 120xc2x0 current application timing, so that the transistor Q3 is turned ON. At this time, the power supply side transistors Q1, Q3 and Q5 are simultaneously turned ON. However, although a current must originally be supplied to the V phase, since the ground side transistors Q2, Q4 and Q6 are all OFF, no currents flow there.
As described above, in the conventional motor driving apparatus, since a square-wave current is supplied to each phase winding of the motor, torque fluctuation caused by a sudden current change is large. Accordingly, the apparatus for reducing motor driving noise has been developed to supply a current continuously changed even at the time of phase switching to each phase winding of the motor 101. However, although this apparatus can reduce the occurrence of vibration caused by a sudden current change, it cannot supply a current to an originally targeted phase because of the short-circuiting of the phases among one another. In other words, the problem of impossible driving has been inherent.
The present invention has been made to solve the foregoing problems, and it is an object of the invention to provide an apparatus for reducing motor driving noise, which is capable of reducing the occurrence of vibration, and preventing the short-circuiting of phases among one another. It is another object of the invention to provide a method for generating a motor driving noise reducing signal, which is executed by a digital circuit not requiring any large-capacity capacitors or the like, yet highly advantageous in cost performance and product yield.
In accordance with the invention, there is provided an apparatus for reducing motor driving noise, comprising: a plurality of switching circuits each including at least a pair of switching elements connected in series between a power supply and a ground; a motor, one end of each of its phase windings is connected to a switching element serial connection point of each of the switching circuits; a noise reducing circuit for generating a current increasing signal and a current reducing signal; a selecting circuit for selecting and outputting the current increasing signal and/or the current reducing signal based on a commutation control signal for changing a polarity at every prescribed electrical angle; a signal synthesizing circuit for synthesizing either the current increasing signal or the current reducing signal selected by the selecting circuit with a current application control signal supplied to a power supply side switching element of the pair of switching elements; a PWM waveform signal generator for outputting a PWM waveform signal based on a current control value; and a PWM synthesizer for synthesizing the PWM waveform signal with a current application direction deciding signal supplied to a ground side switching element of the pair of switching elements.
According to the motor driving noise reducing apparatus of the invention, the noise reducing circuit includes: a current control value output circuit for storing a current control value; a bias time counter for storing a current changing width of an electrical angle 30xc2x0; an arithmetic circuit for obtaining a current amplification factor from the current control value and the current changing width; a noise reducing duty increasing/decreasing timing signal generator for generating a duty control value based on a commutation control signal for reversing a polarity at every electrical angle 30xc2x0, a clock signal and the current amplification factor; and a noise reducing waveform signal generator for generating a current increasing signal for continuously increasing a current for the amount corresponding to the current changing width within an electrical angle 30xc2x0, and a current reducing signal for continuously reducing a current for the amount corresponding to the current changing width within an electrical angle 30xc2x0, based on the commutation control signal, the clock signal, the duty increasing/decreasing signal, and the current control value.
The motor driving noise reducing apparatus of the invention further comprises: position detecting means for comparing a terminal voltage of each phase winding of the motor with a voltage of a middle point at which the phase windings are connected with one another, and detecting the rotational position of the motor; and sensorless driving arithmetic means for outputting a signal that increases a current application width of a currently energized motor winding, based on a detection signal from the position detecting means.
In accordance with the invention, there is provided a method for generating a motor driving noise reducing signal, comprising the steps of: obtaining a current amplification factor by dividing a current changing width decided according to a motor revolution speed by a current control value: adding 1 to a duty control value every time a count value of a reference clock reaches the current amplification factor; determining whether the count value of the reference dock has reached a maximum value of the current amplification factor, until the count value of the reference clock reaches the duty control value; generating a current reducing signal by setting the count value and the current reduction rate to 0, if the maximum value has been reached; determining whether the count value of the reference clock has reached the maximum value of the current amplification factor, until the count value of the reference clock reaches a value obtained by adding to the duty control value the value which is obtained by subtracting the current control value from the upper limit of the current control value; generating a current increasing signal by setting the count value and the current amplification factor to 0, if the maximum value has been reached; and adding the current increasing signal and the current reducing signal to a current application control signal supplied to a power supply side switching element of switching elements connected in series between a power supply and a ground.